1. Field of the Invention
The invention generally relates to systems for transmitting modulated signals, and in particular to the transmitting of television signals. The invention also relates to a single measurement method, and device, for displaying and rating the linearity of a transmission system.
2. Discussion of the Background
An information-carrying signal may be regarded as an amplitude-modulated carrier frequency or as a sum of n spectral components (n.gtoreq.2) brought together in one channel. Such a signal is characterized by two dimensions:
Its spectral breadth, that is to say the interval between the extreme frequencies F.sub.1, F.sub.2 or angular frequencies .omega..sub.1, .omega..sub.2 of the spectrum: this dimension is written [F.sub.1, F.sub.2 ] or [.omega..sub.1, .omega..sub.2 ].
Its peak level, N.sub.c. Periodically all the spectral components reach their maximum amplitude; at the same instant the signal reaches its peak level. This second dimension of the signal is written [-N.sub.c, +N.sub.c ] or [0, .vertline.N.vertline.]. The signal to be transmitted is thus defined by these two dimensions.
Respect for the integrity of this signal, that is to say respect for the information which it contains, is bound by the following condition: the transfer characteristic of the transmission system, that is to say the function f connecting the output voltage V.sub.s with the input voltage V.sub.e, V.sub.s =f(V.sub.e), in the space {[.omega..sub.1, .omega..sub.2 ][0, .vertline.N.sub.c .vertline.]}, that is to say as a function of the angular frequency parameter .omega. and of the level parameter N, must be linear. The derivative of this characteristic, that is to say the transfer function, must be capable of being written: .tau.=.rho..sub.(.omega.) e.sup.j.phi.(.omega.), .sub..rho.(.omega.) and .phi..sub.(.omega.) being the modulus and the phase respectively of the transfer function, with .rho..sub.(.omega.) =constant and .phi..sub.(.omega.) =K.omega.+.phi..sub.0 ; that is to say the transfer function depends only on the frequency and not on the level of the signal.
This is the expression for the transfer function of a linear network. It is then possible to apply the superposition theorem which characterizes the simultaneous transmission of several signals in one channel without these various signals interacting.
However, in an arbitrary transmission system, the modulus and the phase of the transfer function are not dependent solely on the angular frequency .omega., but are also dependent on the level N of the signal. In this case, the transfer function can only be written in the form EQU .tau..sub.(.omega.,N) =.rho..sub.(.omega.,N).multidot.f[.phi..omega..sub.(.omega.,N) ].
It will only be possible to get back to the previous notation when: EQU d.tau..sub.(.omega.,N) /dN=0,
that is to say when simultaneously ##EQU1## This is the linearity condition.
The problem is the checking of this linearity condition through a simple test procedure, then from the observed defects in linearity located on the transfer characteristic V.sub.s =f(V.sub.e), their correcting.
According to the prior art, in the field of television transmission, the use of a test signal, composed of ten luminance levels with a signal of frequency 4.43 MHz or 3.58 MHz (chrominance subcarrier) superimposed on each of these levels, allows the linearity conditions to be checked at two points of the frequency spectrum, .omega..sub.p near the picture carrier frequency, and .omega..sub.c near the chrominance carrier frequency. With the aid of appropriate filters, it is known to display the linearity and the differential gain in the video frequency band, termed the "LF Linearity", and "Differential Gain" and to compare the variation in these parameters between the input of the transmitter or of the retransmitter and the output by way of the demodulator or receiver. The wave modulated by this test signal is the input signal V.sub.e for the transmission system which delivers the wave V.sub.s at its output, the transfer characteristic being the curve V.sub.s =f(V.sub.e). The use of a synchronous demodulator allows the phase of the carrier to be displayed as a function of the level, a parameter termed the "Incoming Phase". A "vectorscope" allows the variation in the phase of the color subcarrier to be tracked as a function of the level: this measurement is the "Differential Phase" measurement.
As indicated above these four measurements "LF Linearity", "Differential Gain", "Incoming Phase" and "Differential Phase" are therefore measurements through sampling for two frequencies characteristic of the spectrum to be transmitted .omega..sub.p and .omega..sub.c. If these four parameters are utilized for what they are, that is to say samples, and if they have not been the subject of special processing operations, it is possible to deduce therefrom by interpolation and extrapolation the linearity from .omega..sub.1 to .omega..sub.2, that is to say throughout the spectrum of the signal.
These measurements can be confirmed and refined, in the case of a single channel sound and picture transmission, when one or two sound channels are added to the modulated picture signal. The nonlinearity in the transfer characteristic causes the appearance of beat products picked up on reception as recurrent noises:
intermodulation PA1 cross modulation PA1 phase noise.
The monitoring of the quality, the adjusting and the maintaining of an amplifying system are therefore, conventionally, complicated operations which require a large number of measuring instruments).